In the process control industry, many process applications may produce unacceptable levels of noise. In control valve applications, valve trim, such as cages, may encounter a variety of harsh operating conditions. For example, in Liquid Natural Gas (LNG) distribution applications, large compressors are used to pressurize the natural gas to liquid phase prior to introduction into a distribution pipeline. It is known that during compressor operation a potentially destructive condition known as “surge” may occur. The surge point of the compressor is generally defined as the operating point where the maximum pressure at minimum stable flow can be achieved for a given compressor speed.
Operation of the compressor at or below the surge point may cause unstable operation that may cause compressor surge to occur. For example, in normal operation as gas flow through the compressor system decreases, the fluid pressure increases to maintain flow, but near the surge point, the compressor cannot impart enough momentum in the gas to continue gas flow through the compressor, causing gas flow to momentarily stop. As flow stops, the inlet pressure falls and the outlet pressure may become greater than the inlet pressure, which causes a flow reversal within the compressor (i.e., gas flows momentarily from the outlet to the inlet). The flow reversal is maintained until an adequate pressure head develops at the turbine inlet to overcome the surge condition. If compressor operation continues near the surge point, the surge condition will repeat, causing repetitive flow reversals, until the process conditions change. The flow reversals associated with compressor surge create compressor thrust reversals that can cause unstable axial and radial vibration that can damage the compressor and create high levels of noise.
To avoid compressor surge from occurring and damaging the compressor, anti-surge systems are built around the compressor. It is commonly known that anti-surge systems require high capacity anti-surge valves (i.e., large flow and high pressure valves). For example, anti-surge valves may have 22 inch ports and operate at a 550 psi pressure differential. One of ordinary skill in the art can appreciate that these flow conditions create high mass flow rates that can produce very turbulent flow and create unacceptable levels of noise. To prevent unwanted noise and damaging vibration, anti-surge valves also rely upon noise attenuating fluid pressure reduction devices (e.g., noise abating trim components). Current noise abating trim components, such as the Whisperflo® trim, available from Fisher Controls International LLC, includes a valve cage using multi-stage fluid pressure reduction designs formed from a stack of annular plates that define multiple restrictive passageways between a hollow center and an outer perimeter. Such devices have been developed for optimal operation in low pressure, mid pressure, and high pressure applications.
In some applications, it is beneficial to have the entire valve cage constructed from the stacked disc assembly such that the stacked disc assembly provides noise abatement and fluid pressure reduction throughout the entire range of travel of the related fluid flow control element. However, in other applications, noise abatement is only required throughout a portion of the travel. In these situations, when the entire valve cage is constructed from stacked discs, the stacked disc assembly actually reduces the potential overall flow capacity of the control valve.